Legend of Duo

It is the early 21st century, and mankind is doomed. Due to a mysterious virus that steals the essence of human life, humanity has been pushed to the brink of extinction with only one hope for salvation: the blood of a vampire. Vampires have closely guarded the key to curing the virus for several years, which has led to an immense rift between them and humans and the world's descension into chaos.

A vampire named Duo, having leaked the secret to saving humanity, is now considered a traitor to his race, and the powerful vampire Zieg is on his trail. However, due to Zieg's sense of duty, his heart is torn: should he kill the man who betrayed his people, or will his personal history with Duo sway him to have a change of heart?

(Source: MAL Rewrite)